The listener interface for receiving WindowEvents, including
WINDOW_GAINED_FOCUS and WINDOW_LOST_FOCUS events.
The class that is interested in processing a WindowEvent
either implements this interface (and
all the methods it contains) or extends the abstract
WindowAdapter class (overriding only the methods of interest).
The listener object created from that class is then registered with a
using the Window's addWindowFocusListener method.
When the Window's
status changes by virtue of it being opened, closed, activated, deactivated,
iconified, or deiconified, or by focus being transferred into or out of the
Window, the relevant method in the listener object is invoked,
and the WindowEvent is passed to it.